[Noisebridge-discuss] driving multiple LEDs with minimal batteries

Corey McGuire coreyfro at coreyfro.com
Tue Jan 4 15:02:42 PST 2011


Want to meet Thursday before our respective meetings and discuss how to
handle it?

I'm an RC car/heli/plane nerd, so I have spent way too much time learning
about batteries from my arm chair.  I am not, however, an EE or a Chemist.

On Tue, Jan 4, 2011 at 2:59 PM, Michael Shiloh
<michaelshiloh1010 at gmail.com>wrote:

> This info needs to be wikified. led and battery questions are easily in
> the top 5 FAQs.
>
> i was about to do it but wonder about the structure.
>
> do we have a technical information category? i couldn't figure out how
> to get a list of categories.
>
> should there be a link to technical information from the front page?
>
> where would you expect to find this sort of information?
>
> On 01/04/2011 02:54 PM, Corey McGuire wrote:
> > Typically with NiCad and NiMH batteries, Sub-C cells are the best bang
> > for buck and have the best energy density.  This is because they are
> > used in just about every industrial/hobbie rechargeable device and
> > battery companies focus on this packaging.  These are the cells roombas
> use.
> > http://www.batteryspace.com/subcsizeseriesbatteries.aspx
> >
> >
> > On Tue, Jan 4, 2011 at 2:49 PM, Corey McGuire <coreyfro at coreyfro.com
> > <mailto:coreyfro at coreyfro.com>> wrote:
> >
> >     The trouble with rechargeable batteries is how you handle over
> >     charging and over discharging.  You want the simplest solution
> possible.
> >
> >     A drawback of LiPO batteries is over discharging.  Other battery
> >     technologies can also be over discharged, but the advantages of the
> >     others is, their voltages drop to the point where the LED's would be
> >     very dim before the cells reached a critical charge level.  LiPO's
> >     typically hold their voltage to the bitter end...at least to the
> >     levels that we humans can detect with our senses.  Motor's will
> >     happily whir, LED's will burn bright, and you won't know it's too
> late.
> >
> >     To prevent over discharge with LiPO's, you need to have a voltage
> >     cut off circuit of some kind.
> >
> >     Using Alkaline cells (AAA, AA, C, D, etc) means people can opt to
> >     use NiMH or NiCad batteries.  Then battery charging is their
> >     problem, and not yours.
> >
> >     If you want to solve the recharging problem, your self, you can
> >     include NiMH or NiCad batteries ( http://www.batteryspace.com/ ) and
> >     just provide a wall wart that gives 1.5v per cell wired in series (2
> >     cells, 3v, etc.) at 50mah-100mah of current, and you won't have to
> >     worry about over charging.
> >
> >     The same can be done for the A123, LiFePo4 cells I linked, only they
> >     require 3.6v per cell at a low current.
> >
> >
> >     On Tue, Jan 4, 2011 at 2:26 PM, Dr. Jesus <j at hug.gs
> >     <mailto:j at hug.gs>> wrote:
> >
> >         On Tue, Jan 4, 2011 at 12:06 PM, meredith scheff
> >         <satiredun at gmail.com <mailto:satiredun at gmail.com>> wrote:
> >          > I'd like to do a soft circuit scarf or three, but I'm always
> >         running up
> >          > against the problem of power. I usually use fairly low power
> LEDs
> >          > (<2v) driven by a 9v battery or one of sparkfun's LiPos.
> >          > I've heard tell of somehow being able to power more, but I'm
> >         still learning
> >          > this EE stuff. Could some kind person point me in the right
> >         direction?
> >
> >         You want to wire them up in parallel:
> >
> >         (+) -|>|- (-)
> >         (+) -|>|- (-)
> >         (+) -|>|- (-)
> >
> >         Not series:
> >
> >         (+) -|>|-  -|>|- -|>|- (-)
> >
> >         If you have too many LEDs on the same battery it won't work
> because
> >         they will draw too much power.  How many is too many depends on
> the
> >         LEDs.  If you hook them up directly to the battery, they may
> >         draw more
> >         current than they're rated for, which is bad for the LEDs and may
> >         cause the lipo battery to catch fire.
> >
> >         The cheap and easy way to make sure they don't draw too much
> >         power is
> >         to put a resistor in series with the LED to limit the current.
> >
> >         (+) -/\/\/\-|>|- (-)
> >         (+) -/\/\/\-|>|- (-)
> >         (+) -/\/\/\-|>|- (-)
> >
> >         The resistor value in ohms is (battery volts)-(LED voltage drop)
> /
> >         (the LED current you want in amps).  If you want 20 milliamps
> >         through
> >         a single 2 volt LED and you're using a LiPo battery:
> >
> >         (4 volts - 2 volts) / 0.02 amps = 100 ohms
> >
> >         The LiPo battery voltage is only 4 volts when it's fully charged.
> >         When it begins discharging, it drops to about 3.7 for most of its
> >         discharge curve and then to 2.7 right at the very end.  Even
> though
> >         the "right" number is 3.7 volts for most of the time the battery
> is
> >         discharging, use 4 volts in your calculations to avoid using too
> >         little resistance and putting too much current through the LED.
> >
> >         If you have too many LEDs in the circuit, the battery will try to
> >         supply too much current.  If the battery is unregulated it might
> get
> >         hot and catch fire.
> >
> >         The resistor "throws away" the extra energy going to the LED in
> the
> >         form of heat, but a resistor is really cheap and you can put lots
> of
> >         them in your circuit easily.  To make the battery last longer,
> you
> >         need to build or buy a constant-current regulator or a switching
> >         regulator, which is harder and a little more expensive.
> >         _______________________________________________
> >         Noisebridge-discuss mailing list
> >         Noisebridge-discuss at lists.noisebridge.net
> >         <mailto:Noisebridge-discuss at lists.noisebridge.net>
> >         https://www.noisebridge.net/mailman/listinfo/noisebridge-discuss
> >
> >
> >
> >
> >
> > _______________________________________________
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>
> --
> Michael Shiloh
> KA6RCQ
> www.teachmetomake.com
> teachmetomake.wordpress.com
> _______________________________________________
> Noisebridge-discuss mailing list
> Noisebridge-discuss at lists.noisebridge.net
> https://www.noisebridge.net/mailman/listinfo/noisebridge-discuss
>
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